1. Field of the Invention
This invention relates generally to spin valve magnetic transducers for reading information signals from a magnetic medium and, in particular, to an improved antiparallel-pinned spin valve sensor, and to magnetic recording systems which incorporate such sensors.
2. Description of Related Art
Computer systems generally utilize auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an "MR element") as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material separated by a layer of non-magnetic electrically conductive material are generally referred to as spin valve (SV) sensors manifesting the GMR effect (SV effect). In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fe--Mn) layer. The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). In SV sensors, the SV effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage. It should be noted that the AMR effect is also present in the SV sensor free layer and it tends to reduce the overall GMR effect.
FIG. 1 shows a typical SV sensor 100 comprising end regions 104 and 106 separated by a central region 102. The central region 102 has defined edges and the end regions are contiguous with and abut the edges of the central region. A free layer (free ferromagnetic layer) 110 is separated from a pinned layer (pinned ferromagnetic layer) 120 by a non-magnetic, electrically conducting spacer 115. The magnetization of the pinned layer 120 is fixed through exchange coupling with an antiferromagnetic (AFM) layer 121. Free layer 110, spacer 115, pinned layer 120 and the AFM layer 121 are all formed in the central region 102. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed over hard bias layers 130 and 135, respectively, provide electrical connections for the flow of the sensing current I.sub.s from a current source 160 to the MR sensor 100. Sensing means 170 connected to leads 140 and 145 sense the change in the resistance due to changes induced in the free layer 110 by the external magnetic field (e.g., field generated by a data bit stored on a disk).
IBM's U.S. Pat. No. 5,206,590 granted to Dieny et al. and incorporated herein by reference, discloses an MR sensor operating on the basis of the SV effect.
Another type of spin valve sensor currently under development is an antiparallel (AP)-pinned spin valve sensor. FIGS. 2A-2B show an AP-Pinned SV sensor 200 which has been a subject of experiment and modeling by the present inventor. SV sensor 200 has end regions 202 and 204 separated from each other by a central region 206. AP-pinned SV sensor 200 further comprises a Ni--Fe free layer 225 separated from a laminated AP-pinned layer 210 by a copper spacer layer 220. The magnetization of the laminated AP-pinned layer 210 is fixed by an AFM layer 208 which is made of NiO. The laminated AP-pinned layer 210 includes a first ferromagnetic layer 212 (PF1) of cobalt and a second ferromagnetic layer 216 (PF2) of cobalt separated from each other by a ruthenium antiparallel coupling layer 214. The AMF layer 208, AP-pinned layer 210, copper spacer 220, free layer 225 and a cap layer 230 are all formed sequentially in the central region 206. Hard bias layers 235 and 240, formed in end regions 202 and 204, provides longitudinal biasing for the free layer 225. Electrical leads 245 and 250 are also formed in end regions 202 and 204, respectively, to provide electrical current from a current source (not shown) to the SV sensor 200. The magnetization direction 265 of the free layer 225 is set to be parallel to the ABS in the absence of an external field. The magnetizations directions 255 and 260 of the pinned layers 212 and 216, respectively, are anti-parallel with each other and are set to be perpendicular to the ABS.
A key advantage of the AP-pinned SV sensor of FIG. 2A is the improvement of the exchange coupling field strength between the AFM layer 208 and AP-pinned layer 210. This improved exchange coupling increases the stability of the AP-pinned SV sensor 200 at high temperatures which allows the use of corrosion resistant antiferromagnetic materials such as NiO for the AFM layer 208.
Despite of its key advantage, there are two major problems associated with the AP-pinned SV sensor of FIG. 2A. First, the exchange coupling field between the AFM layer 208 and the AP-pinned layer 210 is inversely proportional to the magnetic moment difference (net magnetic moment) between the two AP-pinned ferromagnetic layers 212 and 216. However, it is very difficult to control the net moment of the AP-pinned layer 210 (Co/Ru/Co) because of interfacial diffusion and oxidation that takes place at the interface between the NiO AFM layer 208 and the first pinned layer 212 of Co. This interaction between the NiO AFM layer 208 and the Co first pinned layer 212 creates magnetic dead layer at the NiO/Co interface. The interfacial diffusion and oxidation that take place at the aforementioned interface causes a change in the moment of the first pinned Co layer 212 even after the AP-pinned SV sensor of FIG. 2A has been completely built. The change in the moment of the first pinned layer 212 causes the change in the net moment of the AP-pinned layer 210 by factors of 2 to 3 from one wafer to another. Such large variations in the net moment of the AP-pinned layer 210 result in large variations in pinning fields which compromises the stability of the SV sensor 200 as well as the size and symmetry of the signals detected (read) by the sensor.
Second, substantial amount of the sense current flows in the AP-pinned layer 210 due to the fact that cobalt has a low electrical resistivity of about 12 .mu..OMEGA.Cm. TABLE I summarizes the result of a modeling simulation on the SV sensor 200.
TABLE I ______________________________________ AP-PINNED SV SENSOR OF FIGS. 2A-2B Sheet Resistance Sense Current Material Thickness (.ANG.) .mu..OMEGA. cm Shunting (%) ______________________________________ NiO layer 208 400 insulator -- CO layer 212 29 11.6 15 Ru layer 214 6 20 1.25 CO layer 216 24 11.6 12 Cu layer 220 22 2.7 47 NiFe layer 225 72 25 24 TA layer 230 50 200 2 ______________________________________
According to the results summarized in TABLE I, about 28.25% of the sense current flows in the AP-pinned layer 210. Furthermore, about 15% of the sense current flows in the cobalt layer 212 which does not contribute to reading signals from a magnetic disk. Such a large current flow in the cobalt layers and inability to control the net moment of the cobalt layers contributes to smaller GMR coefficient and read signal asymmetry. Smaller GMR coefficient is due to the fact that a sizeable portion of the sense current flows in a layer that does not contribute to the GMR coefficient. Read signal asymmetry is due to the fact that the current field (H.sub.I), demag field (H.sub.Demag) and the ferromagnetic coupling field (H.sub.FC) effects (all in the same direction) on the free layer magnetization (FIG. 2B) are larger than the effect of the AMR on the free layer magnetization direction 265.
Therefore, there is a need for an AP-pinned SV sensor where the amount of current flow in the AP-pinned layer is minimized and the AP-pinned layer has a well controlled net moment.